The use of transmissive optics in directed-energy systems require the knowledge of (1) the real and complex index of refraction, (2) the index and the temperature coefficient of index over the wavelength and temperature ranges for which a system is being designed, and (3) the effect of high-energy optical and nuclear radiation on both the real and complex index of refraction. This paper addresses the current needs for the characterization of transmissive-optical materials and discusses the problems and possible solution involved with meeting these needs.

A comprehensive infrared material database has been created to aid in the development of a rule-based deterministic microgrinding process for infrared optics. The process science effort on infrared materials taking place at the University of Central Florida has compiled a concise collation of easily accessible physical property data to more thoroughly understand the material/processing interactions which occur during optical fabrication. The information exists in a user-friendly form whereby designers and system operators utilizing the Opticam system, can quickly and conveniently access and incorporate material property data into the design and fabrication process. We report on the motivation, organization and application of such a database into the Opticim network. The use of the information in establishment of part design, tooling design, machine parameters and predicting the form, figure and surface quality of the resulting optic is presented.

Building successive generations of state-of-the-art wide field, sub-micron microlithographic lens systems dictates ever-tightening material tolerances that challenge glass manufacturers. This paper discusses the optical material needs for microlithographic lens systems and Tropel's in-house material qualification program. Material qualification is divided into three successive stages: (1) fluorescence testing to qualitatively analyze color center characteristics of the material; (2) homogeneity testing to determine the relative volumetric variations in index; and (3) absolute index testing at multiple wavelengths to determine the material's dispersion characteristics.

The aim of this study was to produce far-IR transmitting chalcogenide glass- ceramics, doped by appropriate nucleants. These materials, with improved thermal and mechanical properties, are to be applied as passive optical components in infrared systems and in high-power laser systems.

The use of chalcogenide non-oxide glasses as infrared optical materials began with the development of a commercial process to produce As₂S₃ glass around 1950. The volume production of FLIR systems in the 80s meant that such glasses were produced annually in `ton' quantities. Methods of production for the most commonly used glasses will be discussed. Optical and related physical properties; absorption, refractive index, thermal change in refractive index and optical homogeneity results for all passive IR materials used in FLIRS will be presented.

Compositions in the MS-In₂S₃ and MS-Y₂S₃ systems (M equals Ca or Sr) have been investigated as potential long-wavelength infrared transmitting materials with possible glass-formability. Previous studies of CaY₂S₄ and SrY₂S₄ single crystals show that these phases have intrinsic infrared cut-offs at longer wavelengths than ZnS, however phases in these systems often exhibit poor crystallinity. Preliminary results indicate that these materials are significantly harder than ZnS, however, they are extremely refractory and therefore difficult to melt by traditional methods.

Fluoride glasses based on ZrF₄ are being developed for various applications including ultralow loss fiber optics and mid-infrared optical components. Advantages of these glasses include a very high transparency from about 225 nm to about 6000 nm. Also, these glasses show a negative temperature coefficient for the change in refractive index so that the thermal optical distortion is very low. The objective of this program is to develop large infrared windows for use with high power mid-infrared lasers. Major problems in preparing these glasses include very low viscosities at the melting and casting temperatures, and high crystal growth rates. Glass preparation techniques have been developed which are suitable for melting and casting quantities of this glass up to 100 kg. Good quality castings have now been made up to 53 cm diameter X 4 cm thick weighing about 40 kg. Finished windows up to 51 cm X 3.8 cm have been made which show few inclusions and low distortion of transmitted light. Glass compositions and melting processes used for the preparation of these disks are described.

A multilayer filter, that will fly on the SOHO mission, has been designed to resist 3 - 4 major solar flares. This paper reports on the effects of the simulated solar flare on the glasses and filters. For the color filter glasses, transmittance drops could be up to 18% at 500 nm, for the 7-year simulated test, while for the heat absorbing glasses it was a few %. The filter transmittance dropped relatively by 2.6% and 15%, for the 2-year and 7- year doses, respectively. In most cases the filter profiles were slightly affected. The effect of the irradiations on the coatings, the adhesive and the glasses is discussed. Guidelines for minimizing effect of irradiations on coatings and glasses are given.

New advances in VUV thin film filter technology have been made using filter designs with multilayers of materials such as Al₂O₃, BaF₂, CaF₂, HfO₂, LaF₃, MgF₂, and SiO₂. Our immediate application for these filters will be in an imaging system to be flown on a satellite where a 2 X 9 R_E orbit will expose the instrument to approximately 275 krads of radiation. In view of the fact that no previous studies have been made on potential radiation damage of these materials on the thin film format, we report on such an assessment here.

In this study we report on the effects of space flight on the reflectance properties of PTFE (polytetrafluorethylene) and aluminum diffuser samples that were flown along with the Shuttle Solar Backscatter Ultraviolet experiment on STS-43, STS-45, and STS-56. These initial experiments showed good stability in the visible and near infrared with some degradation in the ultraviolet which vary from flight to flight which could be attributed to different amounts of contamination levels. This paper reports on the diffuser material processing, the space environment experienced, and the results of the reflectance measurements and the surface chemical analysis before and after flight.

In recent years data have been collected on conductive thermal-control paints, such as PCBZ and NS43G, in order to evaluate their stability to the space environment. In addition to being considered for spacecraft thermal control, the paints have been considered as an alternate material for use within on- orbit calibration systems. This will provide both an absolute calibration, by knowing the magnitude of the reflected light, and flat-field pixel-to-pixel comparisons within an instrument. Data are summarized here, as collected by the Cassini, Multi-angel Imaging SpectroRadiometer, and the Medium Resolution Imaging Spectrometer projects. Properties evaluated include absorptance, reflectance factor, electrical conductivity, thermal cycling, resistance, and environmental exposure stability.

Demands for both superior optical quality and maximum strength in Spinel (MgAl₂O₄) are becoming so stringent that a study has been undertaken to see if varying the processing sequence for Spinel will yield an improvement in its strength without degrading the optical quality. Current ceramic processing includes a hot press cycle followed by hot isostatic pressing to improve optical clarity. Optical fabrication follows, consisting of generation (milling), grinding and polishing. A test matrix, executed utilizing biaxial strength measurements coupled with grain size analysis, has demonstrated that if hot isostatic pressing is delayed until after generation and grinding of the substrate, a substantial increase in strength can be achieved. Knowledge gained in this experiment allows transmission and strength to be optimized to arrive at the best product for a given application.

Two surface modification techniques, a `super polish' (SP), and ion implantation, were evaluated for improving the mechanical performance of sapphire IR window material. Both techniques increased the average strength as measured by 4-point bend tests and were effective in preventing the propagation of surface flaws. Ion implantation improved the reliability at lower stresses more than the SP. Neither process significantly affected the IR transmission. The SP produced a smoother surface as shown by optical scatter measurements.

The advantages and disadvantages of several methods used to prepare gallium arsenide will be discussed relative to the preparation of large plates to be used as IR windows. Amorphous Materials chose to use the Horizontal Bridgman process to prepare plates 4 inch X 9 inch X 0.5 inch. Doped and undoped plates were prepared. Optical and electrical evaluation data will be presented. Scale up to larger sizes will be discussed. A unique process to protect gallium arsenide from rain erosion, provide EMI protection and de- icing capability will be described.

Anticipated airborne targeting, acquisition, and reconnaissance sensor suites will require common, large aperture, highly durable, broadband optical windows. These windows will have apertures as large as 20 inches and will need to transmit from the visible to midwave IR or from the visible to the long wave IR wavelengths. They will also need to survive rain impact at speeds greater than Mach 1.0. Available optical materials do not currently satisfy these requirements. Specific development and improvements will be required. To guide the development efforts, this paper addresses anticipated window requirements, associated improvements that will be required, and potential approaches for achieving them. As a leading manufacturer of optical window assemblies, we are prepared to assume a leadership role in working towards satisfying the optical material challenges addressed in this paper.

Rod shaped silica-poly(vinyl acetate) (PVAc) gels have been prepared by a sol gel process. In situ polymerization of tetraethoxysilane (TEOS) was accomplished in the presence of low molecular weight PVAc by dissolving various amounts of PVAc in a mixture of TEOS, ethanol, water and hydrochloric acid (HCl). Gelation of this mixture was carried out between room temperature and slightly above. Silica-PVAc rods recovered from cylindrical molds were homogeneous and transparent. Gels with weight percents of PVAc ranging from 2% to 50% were prepared. Silica-PVAc gels have higher flexure strengths, less brittle character and improved water durability in comparison with pure sol- gel silica.

A process is presented for fabricating titanium doped silica aerogels. Aerogels were produced by the hydrolysis of TEOS and titanium isopropoxide in ethanol and then supercritically dried in a nitrogen overpressure. A detailed study of gelling and annealing was performed to minimize shrinkage and produce dried gels with densities less than 10% of ordinary glass. Lightweighted structures with densities between 0.16 g/cm³ and 0.06 g/cm³ were produced.

A theoretical model is developed to compute the resulting profile of structures holographically recorded in photoresists. The model takes into account the effects of exposure, photosensitization and isotropy of wet development. The effects of isotropy of wet development, non-linearity of the photoresist response curve, background-light, and the stationary waves produced by reflection at the film-substrate interface are analyzed using the model and the results are experimentally confirmed.

The requirements for superior polychromatic optical performance, low cost, and thermal stability have guided the search at Lockheed for abnormal dispersion optical liquids to be used in combinations with inexpensive, normal dispersion glass in a new generation of lens designs. Our research suggests that an optimal liquid would combine the properties of very high abnormality and dispersion with high viscosity and a low freezing point. A liquid with these properties has recently been developed and its identity and characteristics will be reported. The performance benefits of using this new liquid are then explored through the design of a very high performance F/2.8, 300 mm focal length, telephoto lens for 35 mm cameras.

In this paper, we report on a new research program for optical liquids which is now under way. Refractive indices are measured over the useful transmission range of each liquid, which range from the ultraviolet (UV) through the visible and into the infrared (IR). Many of the liquids have abnormal dispersion and are useful for low-cost materials in lens designs in the visible. In the UV and IR, design options are now possible which are not available with existing solid materials. Liquids have not found widespread application in optical design because the index of refraction of the liquids was not available for more than a very select few, and the data that existed was only in the visible spectral region. Furthermore, the relationships between the optical properties and the physical-chemical structures of the liquids were not available. This was the motivation for our project.

It has been demonstrated recently that high performance apochromatic lenses and objectives can be designed using optical liquids that have abnormal dispersion properties. This has opened the way to low cost apochromatic objectives with optical quality equivalent to lenses using Calcium Fluoride and which are superior in other ways as well (reduced scattering, for example). This paper describes a study on correlating the refractive index dispersion of optical liquids to their chemical structures. The ultimate goal is to establish an understanding of how different chemical moieties affect optical properties of organic liquids.

Several glass blanks have been produced, handled, and machined to their approximate shape before ceramization. The melting and spin casting of glass menisci is completed. One glass meniscus has been ceramized and machined to its final shape. In the present paper the experience gained when performing the different process steps will be presented and discussed. This includes a comparison of the theoretical calculations during the planning phase with actual experimental results. The properties measured so far for the first ZERODUR meniscus all meet the requirements, in many cases surpass them.

This is a review of the demonstrated capability to produce ultra-smooth super-polished surfaces (< 2 angstroms rms) when a reaction bonded silicon carbide (RB SiC) mirror is coated with a SiC coating. Also covered is the viability of this design choice for large scale lightweighted mirror blanks. A super-polished surface finish on bare RB SiC is not attainable due to the difference in hardness of the two phases, SiC and up to 30% silicon. Therefore, deposition of an additional layer of SiC onto the figured RB SiC blank is required. This technique has been demonstrated and can result in meter class optics with a surface roughness in the regime of less than 2 angstroms rms. The blank and optical fabrication techniques are described along with performance characterization data.

The requirement for lightweight mirrors for spaceborne application has often been met through the use of beryllium or low-expansion glass-like materials. The current availability of silicon carbide substrates and the development of suitable manufacturing and finishing process, however, provide an alternative material for such uses. This paper describes the manufacture of a series of lightweight silicon carbide scan mirrors intended for satellite installation. The finishing procedures employed and the test results obtained while establishing surface accuracies on the order of one millionth of an inch are presented and discussed.

In this paper, we report on our work at Lockheed in designing, fabricating, and testing lenses using liquid optical elements to achieve apochromatic color correction without using abnormally dispersive optical glasses and/or calcium fluoride. We have found that designs using liquid optical elements have equivalent or superior optical performance and a cost savings for the lens assembly between 3:1 and 10:1, depending on the diameter. We have also developed designs with apochromatic color correction using plastic and liquid optical elements, a procedure which greatly reduces the weight of the lens as well as its cost. The principle is simple: we eliminate expensive abnormally dispersive glasses and crystals and replace them with inexpensive abnormally dispersive liquids. Designs we have developed include photographic objectives, telescope objectives, and lenses for use in the ultraviolet and infrared spectral regions. An apochromatic telescope objective has been put into production under license from Lockheed and is now available commercially. This paper includes specific design examples.